The present invention is related to the field of keratorefractive surgery (also known as refractive corneal surgery), which involves surgical procedures to alter the shape of the cornea and thereby alter the refractive pattern of light passing through the cornea. A thorough review of keratorefractive surgical procedures is provided by Binder P. S., entitled "Refractive Surgery: Its Current Status and Its Future", in The Contact Lens Association of Ophthalmologists Journal, Vol. 11(4), pp. 358-375 (Oct./Dec. 1985). In particular, the present invention is directed to a surgical instrument for forming incisions in the cornea which is useful, for example, in the treatment of myopia (nearsightedness) or hyperopia (far-sightedness).
Myopia results from physiological imperfections in the eye of a patient which cause the focusing of images in front of the retina. According to the "law of spheres", the human eye can be approximately modeled by a pair of intersecting hollow spheres of different diameter and wall thickness, with the corneal surface being the outer surface of the smaller sphere. Thus, surgical operations to correct myopia have typically involved techniques for effectively flattening the corneal surface in order to cause a posterior displacement of the focal point.
On the other hand, hyperopia results from physiological imperfections causing the focusing of images behind the retina, and a surgical technique intended to correct this condition will necessarily seek to alter the patient's eye so as to achieve an anterior displacement of the focal point.
At the present time, the principal keratorefractive surgical procedure performed (roughly 100,000 procedures annually in the United States) for the correction of myopia is the procedure of Sato, as popularized by Fyodorov and commonly known today as radial keratotomy. In this procedure a series of radial incisions are made in the anterior cornea extending from the edge of the pupil outwardly almost to the limbus. These incisions serve to permanently weaken the cornea so that the effect of the intraocular pressure is to displace the cornea so that the surface of the cornea flattens. However, clinical results with this procedure have been characterized by significant variability of results from patient to patient and an undesirably high level of complications such as under and overcorrection, increased astigmatism,.corneal iron lines and epithelial downgrowth into the incisions. The incisions are left open (i.e. unsutured) after surgery and thus can be susceptible to infection. Inadvertent perforation of the cornea can occur in the practice of this procedure; the eye, once opened, must be closed and infection of the internal matter of the eye, possibly leading to permanent and irrevocable loss of vision, can result from a perforation. If the incision is within the pupillary opening, permanent glare may be experienced by the patient.
An alternative keratorefractive surgical procedure for the correction of myopia is disclosed in U.S. Pat. No. 4,423,728. An annular wedge-like cut is made in (but not completely through) the cornea along a 360.degree. path between the pupil and the limbus. The adjacent walls of the cut are sutured together, causing a carefully controlled flattening of the corneal surface without structurally weakening the cornea or exposing the internal content of the eye to infection. The corrective effect progressively increases as the radius of the wedge-like cut is decreased, and thus virtually all degrees of myopia observed clinically are correctable by this technique. The aforementioned patent also discloses a device for forming such an annular wedge-like cut in a patient's eye in which two distinct support pieces carry respectively a first vertically-disposed blade and a second blade oriented at an acute angle with respect to the first blade. The two support pieces are connected to different portions (180.degree. apart) of a platter which rotates upon a base fixed to the eye. The two support pieces, and thus the two cutting blades, are raised and lowered independently of one another.
The use of the device disclosed in U.S. Pat. No. 4,423,728 has proven to be somewhat problematical. The two blades are adjusted independently and it is quite difficult to position the angled blade at the correct diameter (relative to the diameter of the path of the vertical blade) to create a wedge-like cut of the desired depth and thickness. Furthermore, after the first wall of the cut has been formed the corneal tissue exhibits a tendency to deform around the subsequently passing second blade, so that the second blade merely follows the track left by the first blade and does not cut the intended second wall of the wedge-like cut. Finally, the angled cutting blade cannot be made to follow a non-circular path on the patient's eye.